1. Field of the Invention
This invention relates generally to an extreme ultraviolet (EUV) lithography source and, more particularly, to an EUV source that employs an input laser beam positioned off-axis or asymmetrically relative to the first collection optics to improve the fraction of produced EUV radiation.
2. Discussion of the Related Art
Microelectronic integrated circuits are typically patterned on a substrate by a photolithography process that is well known to those skilled in the art, where the circuit elements are defined by a light beam propagating through a mask. As the state of the art of the photolithography process and integrated circuit architecture becomes more developed, the circuit elements become smaller and more closely spaced together. As the circuit elements become smaller, it is necessary to employ photolithography light sources that generate light beams having shorter wavelengths and higher frequencies. In other words, the resolution of the photolithography process increases as the wavelength of the light source decreases to allow smaller integrated circuit elements to be defined. The current trend for photolithography light sources is to develop a system that generates light in the extreme ultraviolet (EUV) or soft X-ray wavelengths (13.4 nm).
Different devices are known in the art to generate EUV radiation. One of the most popular EUV radiation sources is a laser-plasma, gas condensation source that uses a gas, typically Xenon, as a laser plasma target material. Other gases, such as Krypton, and combinations of gases, are also known for the laser target material. The gas is forced through a nozzle, and as the gas expands, it condenses and converts to a liquid spray. The liquid spray is illuminated by a high-power laser beam, typically from an Nd:YAG laser, that heats the liquid droplets to produce a high temperature plasma which radiates the EUV radiation. U.S. Pat. No. 5,577,092 issued to KUBIAK discloses an EUV radiation source of this type.
FIG. 1 is a plan view of a known EUV radiation source 10 including a nozzle 12 and a laser beam source 14. A gas 16 flows through a neck portion 18 of the nozzle 12 from a gas source (not shown). The gas is accelerated through a narrowed throat portion and is expelled through an exit collimator of the nozzle 12 as a jet spray 26 of liquid droplets. A laser beam 30 from the source 14 is focused by focusing optics 32 on the liquid droplets. The heat from the laser beam 30 generates a plasma 34 that radiates EUV radiation 36. The nozzle 12 is designed so that it will stand up to the heat and rigors of the plasma generation process. The EUV radiation 36 is collected by collection optics 38 and is directed to the circuit (not shown) being patterned. The collection optics 38 can have any suitable shape for the purposes of collecting and directing the radiation 36. In this design, the laser beam 30 propagates through an opening 40 in the collection optics 38.
It has been shown to be difficult to produce a spray having large enough droplets of liquid to achieve the desired efficiency of conversion of the laser radiation to the EUV radiation. Because the liquid droplets have too small a diameter, and thus not enough mass, the laser beam 30 causes some of the droplets to break-up before they are heated to a sufficient enough temperature to generate the EUV radiation 36. Typical diameters of droplets generated by a gas condensation EUV source is on the order of 0.33 microns. However, droplet sizes of about 1 micron in diameter would be desirable for generating the EUV radiation. Additionally, the large degree of expansion required to maximize the condensation process produces a diffuse jet of liquid, and is inconsistent with the optical requirement of a small plasma size.
To overcome the problem of having sufficiently large enough liquid droplets as the plasma target, U.S. patent application Ser. No. (Attorney Docket No. 11-1119), filed Aug. 23, 2000, titled xe2x80x9cLiquid Sprays as the Target for a Laser-Plasma Extreme Ultraviolet Light Source,xe2x80x9d discloses a laser-plasma, extreme ultraviolet light source for a photolithography system that employs a liquid spray as a target material for generating the laser plasma. In this design, the EUV source forces a liquid, preferably Xenon, through the nozzle, instead of forcing a gas through the nozzle. The geometry of the nozzle and the pressure of the liquid propagating through the nozzle atomizes the liquid to form a dense spray of liquid droplets. Because the droplets are formed from a liquid, they are larger in size, and are more conducive to generating the EUV radiation.
Sources for EUV lithography based on laser produced plasma currently employ laser beams that are symmetric with the axis of the first collection optics. Hardware, including the nozzle, diffuser, etc., that provides the target material for the laser beam is positioned proximate the focal point of the first collection optics because the plasma generation area must be located at this position. The nozzle is positioned orthogonal to the laser beam. In this position, the hardware obscures the EUV radiation reflected from the central portion of the optics. This is because the EUV radiation generated from the plasma has an angular distribution that is strongly peaked in the direction of the incoming laser beam and decreases to nearly zero at angles orthogonal to the laser beam. Hence, the region of the strongest EUV illumination at the collection optics cannot reflect to subsequent optics, resulting in a substantial decrease in the fraction of EUV radiation that can be utilized.
FIG. 2 is a schematic plan view of a known EUV radiation source 50 from a different angle than the source 10 shown in FIG. 1 that demonstrates this problem. In this example, a nozzle and associated target production hardware 52 is shown positioned relative to a plasma spot 54. The target laser beam 56 propagates through an opening 58 in collection optics 60, where the axis of the laser beam is symmetric relative to the shape of the optics 60. The collection optic 60 is generally dish-shaped having a reflective surface shape suitable for the purposes described herein. In this configuration, the angular distribution 62 of the produced EUV radiation causes the strongest EUV radiation to propagate towards the collection optics 60 in a direction directly opposite to propagation direction of the laser beam 56. Thus, the stronger EUV radiation reflected from the optics 60 is directed back towards the target production hardware 52 and the weaker EUV radiation is reflected at the edges of the collection optics 60. Thus, the target production hardware 52 blocks much of the strong EUV radiation, which results in a significant loss of this radiation.
What is needed is a design change of the known EUV source that does not obscure a significant portion of the generated EUV radiation so as to increase the fraction of EUV radiation that is usable. It is therefore an object of the present invention to provide such a source.
In accordance with the teachings of the present invention, an EUV source is disclosed that delivers the laser beam to the plasma generation area off-axis relative to the first collection optics. Particularly, the first collection optics has an opening for the laser beam at a location so that laser beam is directed towards the plasma generation area at an angle that is off-axis relative to the collection optics. Thus, the strongest EUV radiation is not blocked by the target production hardware. In one embodiment, the collection optics is a section of a dish, where the direction of the laser beam causes the strongest EUV radiation to be reflected from the outer edges of the optics. In another embodiment, the collection optics is a full dish having two openings for two separate laser beams to generate EUV radiation sent in a direction so it is also reflected at the outer edges of the optics.
Additional objects, advantages and features of the present invention will become apparent to those skilled in the art from the following discussion and the accompanying drawings and claims.